1. Field of the Invention
The present invention relates to an apparatus for detecting, without electrical contact defective insulators in an insulating column supporting an electrical conductor in a power line and wherein the column is formed of a plurality of serially connected insulators.
2. Description of Prior Art
In order to support the electrical conductors in a power line network, porcelain disk-type insulators are connected together to form support columns for the electrical conductors. The number of insulators in the column varies as the function of the intensity of the voltage present on the conductor supported thereby. For example, with a line voltage of 735 KV present on a conductor, there may be required as much as 35 insulators in each column in order to have adequate insulation at all atmospheric conditions for the proper functioning of the power circuit. Often, with aging, some of the porcelain insulators will develop interior fissures thereby short-circuiting themselves. When this occurs, the dielectric property of the column is reduced without any exterior visual signs. If various porcelain insulators, in the same column, develop internal fissures or short circuits, this could cause a failure in the power line.
It is a well known fact that these insulating columns have an electric field developed thereabout and which field has a particular characteristic depending on the length of the column and the intensity of the voltage present on the conductor as well as the geometry and the composition of the structure supporting the conductor(s). Accordingly, there is induced voltage within the column when voltage is present on the conductor and it reduces in intensity along the column as we are further away from the conductor supported thereby to reach a minimum value at a distance of about one third its total length from its attach point to the supporting structure. One conventional method of verifying the operability of these porcelain insulators, and the simplest one, consists in occasionally placing a temporary short circuit across each of the insultators in the column. If the insulator being verified is in a good operative condition, there will exist a difference in potential across the insulator and by placing and removing a short circuit thereacross, by means of a jumper wire, a spark will occur at the end of the jumper wire and produce an audible sparking sound. If the porcelain insulator is short-circuited interiorly by a carbonized fissure, the jumper will not produce any sound or will produce a very faint sound as the potential on each side of the insulators is practically the same. Accordingly, the prior method is entirely dependent upon the audible and visual senses of an operator to detect defective insulators.
The above-described method of the prior art has many disadvantages. For example, it is dependent upon the effective hearing of the person which places the jumper wire across the insulators. Also, this jumper wire is usually supported at the end of a long insulated stick and accordingly, the method of placing the jumper wire in proper position is difficult and slow principally if errors are to be minimized. Further, each insulator within the column must be tested and as mentioned above, often a single column may have thirty-five of these insulators. This prior art method is also subject to error as the operator can make a mistake when counting and identifying each of the insulators in the column. Accordingly, it is likely that a good insulator may be replaced and a defective one left in the column when the results provided by the operator are studied and corrective action is taken. Also, this prior art method presents a danger to both the operator and the power line, for the reason that in a situation where many insulators may be defective in a column an additional short circuited insulator in a column could result in the entire current in the line redirecting itself to ground thus presenting a danger to the operator.
One second conventional method consists in measuring the induced potential across each insulator in the column by means of a probe meter which is used to read voltage across each insulator as described in U.S. Pat. No. 1,542,815, "Instrument for Locating and Observing Defective Insulator Units" by Claudius E. Bennett, June 23, 1925 and Italian Pat. No. 502453, "Apparecchio per individuare gli isolatori guasti o difettosi montati su linee elettriche in tensione" by Cesare Castelli, Nov. 30, 1954. The main disadvantage of these apparatus is that the two probe tips have to be placed across each insulator in the column which is difficult and time consuming on long columns: moreover, reading the meter with the unaided eyes is also very difficult.
One third method consists in measuring leakage resistance of each insulator in the column. It is usually done by applying a DC voltage across the insulator and measuring the resulting current. One refined example of this technique is described in U.S. Pat. No. 4,266,184, "Method and Apparatus for Testing Insulators" by Clifford W. Devine. The main disadvantage of this method is again positioning the probe tips across each insulator.